Conventionally, for the purposes of improving fuel efficiency and reducing an environmental load of automobiles, there have been developed automatic idle-stop systems for automatically performing an idle stop with the satisfaction of predetermined conditions. Among the automatic idle-stop systems, an automatic idle-stop system using a starter is advantageous in that a change in system of a vehicle is small and cost is low, while having a problem in that meshing engagement cannot be achieved before an engine is completely stopped.
To cope with the problem described above, there exists an automatic idle-stop system in which a starter motor rpm is brought into synchronization with an engine rpm by energization with speed governing at the time of restart of the engine and a pinion gear is then pushed out to achieve the meshing engagement at the time when an rpm difference becomes smaller than a predetermined threshold value (for example, see Patent Literature 1).
In addition, there is another automatic idle-stop system which predicts a future ring-gear rpm to predict a time at which a pinion gear rpm comes into synchronization with the future ring-gear rpm, so as to control a push-out timing or push-out speed of the pinion gear for synchronization with the above-mentioned time (for example, see Patent Literature 2).